WeakHashMap不会删除过时的条目

Question

How can I model the removal of entries in WeakHashMap if there are no active references to one of it keys. I have next code:

WeakHashMap<Integer, String> weakMap = new WeakHashMap<Integer, String>();
Integer st1 = 5;
Integer st2 = 6;
String val = "BB";
weakMap.put(st1, "AA");
weakMap.put(st2, val);
st1 = 10;
//st1 = null;
//System.gc();
for (Map.Entry<Integer, String> entry : map.entrySet()) {
   System.out.println(entry.getKey() + "  " + entry.getValue());
}

The output is always

6  BB
5  AA

But I expect to get only 6 BB Even if I decomment commented lines it still produce the same output. As I understand if the key in WeakHashMap has no active reference somewhere else outside this weakHashMap the entry with the specified key has to be removed. Am I right? If no, please, suggest the right solution.

Answer 1

Your keys are never garbage collected because Integer s from -128 to 127 are cached (assuming Integer.valueOf is used, which it is for autoboxed int s). You could use values outside that range or use Integer st1 = new Integer(5) to ensure you aren't using cached objects.

Answer 2

The Integer objects from -1000 to 1000 (or somewhere thereabouts) are interned. This means that autoboxing and valueOf() return an object which is stored internally in Integer and hence never garbage collected. You will see the intended behavior if you do this:

Integer st1 = new Integer(5);
Integer st2 = new Integer(6);

...

st1 = 10;

System.gc();

...